1. Field of the Invention
The present invention relates to a threaded tube joint, particularly for joining pipes used in the oil and gas extraction industry. Such pipes can be employed both as pipelines for pumping gas or oil and as linings of the wells themselves.
2. Brief Description of the Prior Art
The assembly of pipes or pipe segments to form strings used in the oil and gas industry is commonly performed using threaded joints. Most of the applications are covered by the standard connections described and specified by the American Petroleum Institute (API). These connections have limitations when subjected to extreme loading conditions that are increasingly common in oil and gas wells, especially those related with high gas internal or external pressure where the risk of leaks shall be eliminated, while resisting at the same time high mechanical loading conditions caused by tension, compression, bending or torque.
One solution provided to improve the seal resistance for oil and gas joints is the metal-to-metal seal placed at various points of the joint, depending on the joint design selected. This type of seal in a threaded joint provides a barrier to gas or liquid pressure while the thread provides mainly for the satisfaction of mechanical requirements and stabbing and running characteristics. Examples of these types of joints are the API Extreme-line. Several seal design alternatives on metal-to-metal seals have been proposed in the prior art. One of the metal-to-metal seal solutions is the one that comprises a frusto-conical annular outer seal surface located in the end of the male member of the joint, which is in contact with a corresponding frusto-conical annular inner seal surface located on the female member of the joint. Due to the tapered seal surface and the definite final assembling position, a radial interference between both surfaces is developed, which by promoting high contact stresses is a guarantee of the seal.
An important technical feature of frusto-conical sealing surfaces is the development of highly concentrated contact stresses on the edge formed by the intersection of the frusto-conical sealing surface of the male member and the cylindrical surface comprised between the seal surface and the threaded zone. The stress distribution along an axial section of a joint of the state of the art is shown in detail in FIG. 6.
The contact stress concentration can be calculated through numerical analysis, and is originated in the stiffness differences between male and female members of the joint that provoke the relative rotation of male member end. The distribution of contact stresses along the seal surface is also affected by the different loading conditions to which the joint is subjected: e.g. tension, compression, internal pressure, external pressure. The coming into existence of high contact stresses during make up, which are normally higher than nominal material yield strength, is a clear risk for galling occurrence in the joint.
When a torus-cone seal combination is used, the contact stresses on the seal surface present a distribution described by a Hertz-type function. The maximum contact stresses and the length of effective contact are related to the intensity of the contact force and the radius of the toroidal surface. The intensity of the contact force could be related to the geometrical interference between male and female sealing surfaces and the dimensions of the male member and female member of the joint.
The patent document U.S. Pat. No. 4,623,173 describes a threaded joint with a toroidal sealing surface of radius larger than 100 mm. Depending on the particular joint dimensions, i.e. its diameter and wall thickness, a sphere-cone seal with high sphere radius could present contact stresses at the seal not high enough to ensure sealing function in operation, increasing the risk of leaks under extreme conditions. On the other hand, if one wants to obviate to this drawback, the usual way to improve seal contact stresses is through the increase of the diametric interference, but this increases the level of mean stresses acting inside the joint, as well as the sliding distance travelled by the male and female members during the make up process, increasing the risk of galling.
On the other extreme, the use of a very small sphere radius is advantageous from a maximum seal contact pressure point of view, but it increases the risk of galling of the joint due to this particularly high contact stress generally higher than material yield strength. Another disadvantage of this alternative is the reduced area of the seal contact surface that could increase the risk of leaks. The non-perfect surface condition and the presence of roughness, lubricant, solid particles, etc. require a minimum length of contact with contact pressure above a minimum value.
In these joints the preferred threads are with trapezoidal profile to increase thereby the load capacity of the joint itself in case of use in very deep wells, and of the Extended Reach Wells type with very important horizontal displacements. To increase the structural resistance of the thread, several designs of joints are using a flank-to-flank contact concept, in which both the stab flank and the load flank of the threads of one of the two members are in contact with the corresponding flank of the other member during and at the end of the assembling.
One main drawback of using trapezoidal threads concept is that the contact on the stab flanks and load flanks during make up increases the risk of galling as well as the torque required, rendering the assembling operation unstable and reducing the re-utilization possibility of such joints.